The present invention relates to a protective film transfer sheet for photo masks. In particular, it relates to a transfer sheet for transferring a protective film suitable for protecting copies for photomechanical process or copies of printed wiring boards, which are susceptible to damage, and a method for transferring a protective film using the transfer sheet.
A surface protective film comprising a thin plastic film such as a polyethylene terephthlate film, as a support, and a releasing film laminated on the plastic film via a self-adhesive or bonding agent has been used for protecting copies of photomechanical process or printed wiring boards, which are susceptible to damage. When the surface protective film is used for photo masks, the releasing film is peeled off from the support and the surface applied with the self-adhesive or bonding agent is laminated on the image surface of photo masks.
In recent years, patterns formed on photo masks, which are copies used for photomechanical process or copies of printed wiring boards, become complicated and are required to have a high resolving power. This raises demand for thinner protective films. However, if the thickness of the plastic film of surface protective films having the aforementioned structure becomes thin, its workability of laminating on photo masks becomes bad, and wrinkles and cells are likely to be produced.
In order to improve such protective films, Japanese Utility Model Publication No.6-20601, for example, discloses a multiple layer sheet for making a protective film which comprises a plastic film having an uneven surface, an abrasion resistance film formed on the uneven surface, an easy-releasing sheet having a self-adhesive layer and a releasing layer formed on the abrasion resistance film. Another transfer sheet comprising a releasing sheet, a resin layer cured by exposure to UV or electron beam and an adhesive layer consisting an ionizing radiation curable resin formed on the releasing surface of the releasing sheet in this order is also known (Japanese Patent Un-examined Publication No. 63-132097).
However, when the generally used self-adhesive is employed as in the disclosure of Japanese Utility Model Publication No.6-20601, the transferred protective layer is insufficient in resistance to marring even if itself has a sufficient hardness because of softness of the self-adhesive layer. In addition, if it is used for protection of photo masks of printed wiring boards, there arises a following problem. That is, photo-resist may stick to the surface of the protective layer and be removed using organic solvents. However, the protective film has low resistance to solvents and, therefore, has low durability.
The adhesive layer consisting of an ionizing radiation curable resin as disclosed in Japanese Patent Un-examined Publication No. 63-132097 is likely to lose adhesiveness when the thickness becomes thin. If the protective film with extremely lowered adhesiveness is used for protection of a photo mask of a printed wiring board, the adhesive layer comes away from the image surface of the photo mask when the photo mask is peeled from the photo-resist on the board. Thus, the protective layer has low durability. Further, if the adhesive layer consists of an ionizing radiation curable resin alone, flowability of the adhesive layer becomes too high, and workability becomes bad because the adhesive layer is likely to be pressed out upon adhering.
The present invention aims at solving the above-mentioned problems. An object of the present invention is to provide a protective film transfer sheet for photo masks capable of providing a protective film, which is excellent in adhesiveness to the image surface of photo masks, mar resistance and resistance to solvents when transferred to photo masks. Another object of the present invention is to provide a method for transferring a protective film using the transfer sheet, which is capable of providing a protective film excellent in adhesiveness to the image surface of photo masks, mar resistance and resistance to solvents when transferred to photo masks.